View Full Version : Honey I blew up the mini lathe speed control board

02-03-2010, 07:12 PM
I bought this board to fit into a mini lathe I bought (which originally had a blown PCB too!)

John S identified this particular board as a potential replacement for the origina (on Ebay- it only cost 15! The board is a Danfoss s100 -often rebadged...varispeed seems to be a constant when rebadged)

I adpated to fit into my Sieg C2 300A ...it's worked fine for about 5-6 months but a couple of nights ago while running at a relative high RPM, a flash eminated from the lathe's control box - fuse blown. Popped in another fuse (not expecting much!)...the motor turned very slow (like 1-2 RPM!) ...and then I smelt that all too familiar smell of some silicon cooking.

For an output stage, the circuit has two TO-220 types with S6060L (SCRs?) & 3 other D6020L (diodes)- no obvious burn marks/melting anywhere, so I'm hopeful it's just a matter of whipping out the larger silicon & finding one that's blown.

http://img684.imageshack.us/img684/236/boardt.th.jpg (http://img684.imageshack.us/i/boardt.jpg/)

Would anyone have a suitable circuit where I could gen up on what the gig is? (not sure if this PWM derived speed control or not?)

At 70 notes for a replacement(& this is the second one that's blown to date on this particular lathe) - this mini lathe pastime is starting to lose it's appel!

02-03-2010, 08:45 PM
I'm guessing you meant S6006L which are Teccor 6A 600v isolated tab SCR (the 6060 is a 60A device and is definitely not in a TO220 package!) and the D6020L is a 600v 20A isolated tab diode also from Teccor. Shouldnt be too hard to work out the circuit, its going to be a full-wave half controlled bridge (2 diodes, 2 SCR arranged as a bridge rectifier) the third diode is a snubber across the motor. What other parts are there on the board?

02-03-2010, 09:55 PM
Hi Irving - tks for the clarification!

Have you any schematic links for the full wave controlled bridge you mention? I'm hopeful there's nothing majorly wrong here, as there's no 'toast' or anything melted (whereas, the original board I spoke of when I bought was absolutely nuked!)

Ok, the board was riveted to that metal surround (as were those SCRs and diodes for heatsinking)- drilled the rivets out - alas, a good few components underneath. I have my first 'lead' though. There's a small 78L08 regulator on the top side of the board ....it is *very* hot (you can see it in the photo I posted aboe - to the left of the piccie next to R33). The DC voltage on its input pin is near 10V...output pin is just 2V! It could be something on it's output dragging the voltage down (hence it getting hot) down so I'll remove it from circuit & see if it checks out. Alas I don't have an 8V regulator handy if it is this ......just 20p from Farnell (but minimum order of 20!)

http://img189.imageshack.us/img189/5848/underneaths.th.jpg (http://img189.imageshack.us/i/underneaths.jpg/)

(a couple of LM324s and one LM339 you see there)

Edit: Ok, i whipped thAT 78L08 out - boshed it into a breadboard that I already have in place with a 16V unregulated DC as an input- it literally smoked! (which suggests it was flakey as a 16V input it should be able to handle - http://www.datasheetcatalog.org/datasheet/WINGS/78L08.pdf ). Ok, so I'll need to source a replacement now!

02-03-2010, 11:08 PM

I order a lot from Farnell and need to put an order in this week, so I'll get you a couple - PM me your address and let me know if there's anything else you need. I thought I had one here but I can't find it.

So you have a quad comparater, probably for zero crossing detection, and a pair of quad opamps, probably acting as buffers for RC timing networks to give start up and stop acceleration/deceleration control. I am guessing the transformer is to provide some power to these and a source of isolated AC for the zero crossing detection, wonder how they drive the SCR gates. All the circuitry is operating relative to one side of the motor and floating relative to supply, unless theres a pulse transformer or opto-isolator on there and I dont see it... so be very careful when putting fingers near there...

The picture shows an ouitline of the circuit - there are several ways to do this but it should help sketch out the main arrangement...


02-03-2010, 11:18 PM
Hi Irving,

Thanks for the offer - there's some relatively hard to source electronics bits & bobs stuff I've been waiting to get from Cricklewood...so I might as well bite the bullet & get ordering from them & lob in a couple of 78l08s too - they've no minimum order, but 3 P&P (not bad & saves me having to drive over there!)

Thanks for the basic outline diagram too.

Re getting my finger too close to the 'big wiggly amps' - yep, lots of high voltages on this board, but not overly worried (originally trained in fixing valve based radar kit by the RAF many moons ago...I've had -500V DC across me a few times while stooping in a darkened ops room fixing radar consoles ...enough to be careful nowadays!)

02-03-2010, 11:28 PM
mmmm been there, done that... 12kV radar output stages :eek:

worse IMHO is the 48v, 500A primary busbar in certain large aircraft, or the 50v similarly rated one in telephone exchanges... wont kill you with volts, but will melt your watchstrap through your wrist if you are stupid enough to wear one and let it short the busbar (seen it done)

04-03-2010, 06:22 PM
Ok, my replacement 78L08 arrived - just slapped on it in. Now there's 8V on it's output, but when I connect my lathe spindle up to the board - instead of not turning at all - t's going at full tilt now (ie speed control has gone), which to me suggests that maybe an SCR is short? (ie rather than being gated, it's 'on' all the time?).

Else I guess it could be the SCR 'gating' waveform up the swanny. I'm figuring the gate voltage on these SCRs is relatively low voltage scopeable with a 10X probe? (My USB scope spec is 35V input max)

Edit: Ok, whipped one of the SCRs out & it checks out a little strange...

http://img22.imageshack.us/img22/589/6020le.jpg (http://img22.imageshack.us/i/6020le.jpg/) (that's a german datasheet, hence the K for cathode!)

A quick refresher on how to test these puppies is here...


which suggests the only low reading I should get is between the cathode & gate (& in one way only?) - well I'm getting 45 ohms between the cathode & gate with DVM probes connected either way.

Also these SCRs are getting quite warm without a load being connected - methinks they might be duff too - will order some up.